Effective Thermal Neutron Collimation for Neutron Capture Therapy Using Neutron Absorption and Scattering Reactions

Abstract

The thermal neutron field available at the Heavy Water Facility of Kyoto University Reactor (KUR) is usually isotropic and the field size is larger than the target. For this reason, collimation of thermal neutrons is needed for clinical irradiations^1,2. The collimator materials chosen to minimize the secondary gamma rays, are lithium-6 used as a thermal neutron absorber, and graphite used as a thermal neutron scatterer. However a collimator, a few cm in diameter using neutron absorbing materials significantly reduces the neutron flux and increases the clinical irradiation time. The shorter irradiation time nevertheless is desirable for patients. In order to reduce irradiation times for patient treatment, we have manufactured a cone shaped thermal neutron collimator using polyethylene which has a high scattering cross section. The characteristics of cone shaped collimators were determined by experiments and calculations. It is noted that the technique of thermal neutron irradiation for neutron capture therapy should be changed according to the characteristics of thermal neutron beam components and also the depth of the tumor.